The present invention relates to a heterojunction bipolar transistor which includes a base layer containing a SiGeC layer with a small degree of lattice strain.
Conventionally, heterojunction bipolar transistors (HBT""s) have a heterojunction barrier formed on the boundary between the energy bands of the two semiconductor materials differing in band gap from each other at a junction between the emitter, the base and the collector for the purpose of improving carrier accumulation and a current amplification ratio. Such HBT""s have come to be used as an active device in a microwave and millimeter wave bands by making use of their high frequency characteristic. Above all, HBT""s using a semiconductor of a Group III-V compound such as GaAs have been studied and developed most energetically; however, in recent years, HBT""s using SiGe material, which is a Group IVxe2x80x94IV compound and can be formed on a silicon substrate are being drawn attention (SiGe-HBT""s). These SiGe-HBT""s are being drawn attention also because the base layer made of a SiGe layer with a narrow band gap makes them operable at a lower voltage than Si-BJT""s.
SiGe-HBT""s proposed so far to achieve speedups are classified into two typical types: the one with a SiGe base layer having a graded composition where the Ge content is gradually increased in the direction from the emitter layer side to the collector layer side (Reference Document 1) (L. Harame et al., xe2x80x9cOptimization of SiGe HBT Technology for High Speed Analog and Mixed-Signal Applications,xe2x80x9d IEDM Tech. Dig. 1993, p.71), and the other with a base layer having a high Ge content and a high concentration of impurity doping so as to make the base layer extremely thin (Reference Document 2) (A. Schuppen et al., xe2x80x9cEnhanced SiGe Heterojunction Bipolar Transistors with 160 GHz-fmax,xe2x80x9d IEDM Tech. Dig. 1995, p. 743.).
In the former transistors provided with the base layer having a graded composition, the graded composition develops an electric field, which facilitates carriers injected in the base layer to drift the base layer. The drifting of the carriers due to the drift electric field is higher in speed than carrier diffusion, so that the time required to transit the base layer (base transit time) is accelerated to provide a high frequency transistor.
On the other hand, the latter heterojunction bipolar transistors have a base layer composed of SiGe having a uniform composition with a high Ge content and having a narrow band gap. The base layer is doped with a high-concentration impurity for carriers in order to decrease its thickness while suppressing a punch through between the emitter and the collector, thereby to accelerate the base transit time. In this case, the base layer having a narrower band gap than the emitter layer reduces the built-in potential of the PN junction between the emitter and the base, thereby achieving a large collector current and a high frequency characteristic at a low voltage.
However, these prior art heterojunction bipolar transistors have the following inconveniences.
First, in the heterojunction bipolar transistors having the graded composition base structure, the gradient of the composition must be large enough to have a large drift electric field. In otherwords, of the base layer, the region in contact with the emitter layer must have a small Ge content, and the region in contact with the collector layer must have a large Ge content. For this, the region of the base layer that is in contact with the emitter layer is generally made from Si only, without Ge. Since the PN junction between the base and the emitter in this case is a homogeneous junction between silicon and silicon, low-voltage operation cannot be expected. In addition, further acceleration of the base transit time for the improvement of the high frequency characteristic requires to further increase the Ge content in the region of the base layer that is in contact with the collector layer; however, when the Ge content is too large, the difference in lattice constant between Si and Ge (lattice mismatch) in the SiGe layer formed on the Si substrate causes dislocations in the base layer, deteriorating the reliability. This indicates that there are limits on an increase in the Ge content. According to Reference Document 3 (S. R. Stiffler et. al., xe2x80x9cThe thermal stability of SiGe films deposited by ultrahigh-vacuum chemical vapor deposition,xe2x80x9d J. Appl. Phys., 70 (3), pp. 1416-1420, 1991.), the upper limit for a practical Ge content in the base layer of a heterojunction bipolar transistor is around 10%. Therefore, under the present circumstances, it is difficult to increase the gradient of the composition of the base layer in order to provide a transistor with a higher frequency or a lower voltage.
On the other hand, the heterojunction bipolar transistors with the uniform composition base structure also have the issue of the critical thickness of the base layer, which causes dislocations due to the above-described lattice constant difference. In reality, the SiGe-HBT having a high Ge content shown in Reference Document 2 suppresses the occurrence of dislocations by not using a process requiring a high temperature treatment during fabricating. Therefore, a silicon process requiring a high temperature treatment cannot be applied, making it impossible to realize a mixed device like a BiCMOS device or an integrated circuit. As a result, there are limits on achieving lower-voltage operation by further reducing the built-in potential.
The object of the present invention is to provide a heterojunction bipolar transistor which can operate at a low voltage and a high speed while maintaining high reliability by providing a means for reducing the amount of lattice strain in the base layer even when there is a large difference between the average band gap of the collector layer and the emitter layer, and the band gap of the base layer.
The heterojunction bipolar transistor of the present invention comprises a first semiconductor layer made from semiconductor material containing Si as a component; a second semiconductor layer made from semiconductor material containing Si, Ge and C as components, having a band gap narrower than the first semiconductor layer and consisting of a top layer, a center layer and a bottom layer; a third semiconductor layer made from Si as a component, and having a band gap wider than the second semiconductor layer stuched in this order onto a substrate; and a heterojunction barrier provided between the first semiconductor layer and the second semiconductor layer, and further comprises: a collector layer formed in the first semiconductor layer and containing a first conductive impurity; a base layer formed in the second semiconductor layer and containing a second conductive impurity; and an emitter layer formed in the third semiconductor layer and containing a first conductive impurity, the second semiconductor layer having an average lattice strain of 1.0% or less.
By controlling the Ge and C contents in the second semiconductor layer represented by, for example, Si1xe2x88x92xxe2x88x92yGexCy where x is the Ge content and y is the C content, it becomes possible to realize low-voltage operation due to a reduction in the built-in potential of the PN junction between the emitter and the base, and to improve operation speed due to the graded composition base structure. In that case, unlike the SiGe layer epitaxially grown on the Si layer, there is no strict upper limit for the Ge content to prevent lattice defect resulting from lattice mismatch. In other words, in the second semiconductor layer including Si, Ge and C as its components, the difference in band gap between the second semiconductor layer and the first and third semiconductor layers can be enlarged, while the average lattice strain, which results from the lattice mismatch with the first and third semiconductor layers that are made from Si and other materials, is restricted to 1.0% or less. As a result, highly reliable and functional heterojunction bipolar transistor can be obtained.
In the heterojunction bipolar transistor, when the second semiconductor layer has undergone a compressive strain, the difference in band gap between the second semiconductor layer made from SiGeC and the first semiconductor layer can be sufficiently large while the C content is reduced. This secures reliability and improves functions as well.
In the heterojunction bipolar transistor, when the band gap of the second semiconductor layer is 1.04 eV or less, it is sufficiently different from the band gap of Si, that is, 1.12 eV or less, thereby providing the same advantages as above.
In the heterojunction bipolar transistor, the first semiconductor layer is made of Si single crystal; and when the second semiconductor layer has a composition represented by Si1xe2x88x92xxe2x88x92yGexCy where x is the Ge content and y is the C content, the composition is in a region surrounded by the following four straight lines:
straight line {circle around (1)}: y=0.122xxe2x88x920.032
straight line {circle around (2)}: y=0.124x+0.028
straight line {circle around (3)}: y=0.2332xxe2x88x920.0233 (C content is 22% or less)
straight line {circle around (4)}: y=0.0622x+0.0127 (Ge content is 22% or less)
on two-dimensional rectangular coordinates whose horizontal axis and vertical axis indicate the Ge content and the C content, respectively. As a result, the lattice strain is restricted to 1.0% or less.
In the heterojunction bipolar transistor, when the center layer of the second semiconductor layer has a uniform composition, a large difference in band gap is secured between the second semiconductor layer and the first and third semiconductor layers.
In the heterojunction bipolar transistor whose center layer has a uniform composition, when the C content in the top layer of the second semiconductor layer increases in the direction from the third semiconductor layer to the center layer, the band structure changes smoothly with almost no band offsets like notches in the emitter-base junction, which provides the heterojunction bipolar transistor with excellent high frequency characteristic.
In the heterojunction bipolar transistor whose center layer has a uniform composition, when the C and Ge contents in the top layer of the second semiconductor layer, which is arranged between the center layer and the third semiconductor layer, increase in the direction from the third semiconductor layer to the center layer, a band structure which changes further smoothly at the emitter-base junction can be obtained.
In the heterojunction bipolar transistor whose center layer has a uniform composition, when the C content in the bottom layer of the second semiconductor layer decreases in the direction from the center layer to the first semiconductor layer, the band structure changes further smoothly with almost no band offsets like notches in the emitter-base junction.
In the heterojunction bipolar transistor whose center layer has a uniform composition, when the C and Ge contents in the bottom layer of the second semiconductor layer decrease in the direction from the center layer to the first semiconductor layer, the band structure changes further smoothly at the emitter-base junction.
In the fundamental structure of the heterojunction bipolar transistor, the band gap in the center layer of the second semiconductor layer decreases in the direction from the third semiconductor layer to the first semiconductor layer, the transit of the carriers in the base layer is stimulated by an electric field, which accelerates the base transit time, thereby providing the high-speed heterojunction bipolar transistor.
In order to decrease the band gap of the second semiconductor layer in the direction from the third semiconductor layer to the first semiconductor layer, the following structures are available.
The third semiconductor layer may be exclusively made from Si; the top layer of the second semiconductor layer may have a composition which changes contiguously to the center layer, and the portion of the top layer that is in contact with the third semiconductor layer may be exclusively made from Si; and the center and top layers of the second semiconductor layer may have a graded composition where at least one of the Ge content and the C content increases in the direction from the third semiconductor layer to the first semiconductor layer.
In that case, in the center and top layers of the second semiconductor layer, the Ge and C contents increase while the ratio between these contents is kept constant.
In the heterojunction bipolar transistor whose second semiconductor layer has a graded composition, when the C content or the C and Ge contents in the bottom layer of the second semiconductor layer decrease in the direction from the center layer to the first semiconductor layer, the band structure changes smoothly with almost no band offsets like notches in the emitter-base junction as mentioned above.
In the heterojunction bipolar transistor whose second semiconductor layer has a graded composition, when the top layer of the second semiconductor layer is made from Si and contains at least one of Ge and C, either the Ge content or the-C content may be changed in the direction from the third semiconductor layer to the first semiconductor layer.
Structures having such a graded composition-are as follows.
When the center layer of the second semiconductor layer has a composition which undergoes a compressive strain, there is a graded composition where the Ge content increases in the direction from the third semiconductor layer to the first semiconductor layer, while the C content is kept constant; there is another graded composition where the C content decreases in the direction from the third semiconductor layer to the first semiconductor layer, while the Ge content is kept constant; there is further another graded composition where the Ge content increases and the C content decreases in the direction from the third semiconductor layer to the first semiconductor layer; and there is further another graded composition where the Ge content and the C content increase in the direction from the third semiconductor layer to the first semiconductor layer.
When the center layer of the second semiconductor layer has a composition which undergoes a tensile strain, there is a graded composition where the Ge content decreases in the direction from the third semiconductor layer to the first semiconductor layer, while the C content is kept constant; there is another graded composition where the C content increases in the direction from the third semiconductor layer to the first semiconductor layer, while the Ge content is kept constant; there is further another graded composition where the Ge content decreases and the C content increases in the direction from the third semiconductor layer to the first semiconductor layer; and there is further another graded composition where the Ge content and the C content increase in the direction from the third semiconductor layer to the first semiconductor layer.
By providing the center layer of the second semiconductor layer with a graded composition within the region having either a compressive strain or a tensile strain, the SiGeC content changes without passing through the region where the center layer made from SiGeC is lattice-matched. This can avoids inconveniences like a reverse gradient of the band gap in the center layer of the second semiconductor layer.
In the heterojunction bipolar transistor where the center layer of the second semiconductor layer has a graded composition, it is preferable that either the C content or the C and Ge contents in the top layer of the second semiconductor layer increases in the direction from the third semiconductor layer to the center layer.
It is also preferable that either the C content or the C and Ge contents in the bottom layer of the second semiconductor layer decrease in the direction from the center layer to the first semiconductor layer.
The method for fabricating a heterojunction bipolar transistor of the present invention comprising: process (a) for forming, on a first semiconductor layer of first conductivity type which contains Si as a component and functions as a collector layer, a second semiconductor layer containing a SiGeC layer and having a narrower band gap than the first semiconductor layer and an average lattice strain of 1.0% or less; process (b) for forming a third semiconductor layer containing at least Si and having a band gap wider than the second semiconductor layer onto the second semiconductor layer; process (c) for forming a conductive layer containing a first conductivity type impurity which is in contact with a part of the third semiconductor layer; process (d) for forming a base layer by introducing a second conductivity type impurity at least to a part of the second semiconductor layer; and process (e) for forming an emitter diffusion layer by diffusing the first conductivity type impurity in said conductive layer into the third semiconductor layer by a heat treatment.
According to this method, it has been confirmed that in fabricating the heterojunction bipolar transistor, when the crystalline of the second semiconductor layer including Si, Ge and C has an average lattice strain of 1.0% or less, the excellent crystalline is maintained after the heat treatment conducted in the process (e). Thus, the diffusion of the first conductivity type impurity from the conductive layer makes it possible to provide the emitter diffusion layer only at the local portion of the third semiconductor layer, thereby achieving a heterojunction bipolar transistor with excellent electric properties including a high frequency characteristic.
It is preferable that a Si layer is used for the first semiconductor layer, and in said process (a), a Si1xe2x88x92xxe2x88x92yGexCy layer (where x is the Ge content and y is the C content) is formed as the second semiconductor layer, and in said process (b), a Si layer is formed as the third semiconductor layer.
It is also preferable that in the process (a), the second semiconductor layer is formed to have a composition in the range surrounded by four straight lines as follows on two-dimensional rectangular coordinates whose horizontal axis and vertical axis indicate the Ge content and the C content, respectively;
straight line {circle around (1)}: y=0.122xxe2x88x920.032
straight line {circle around (2)}: y=0.1245x+0.028
straight line {circle around (3)}: y=0.2332xxe2x88x920.0233 (Ge content is 22% or less)
straight line {circle around (4)}: y=0.0622x+0.0127 (Ge content is 22% or less).
In the process (b), the first conductivity type impurity is doped in the third semiconductor layer concurrently with epitaxial growth in order to increase the concentration of the impurity in the third semiconductor layer in addition to the introduction of the impurity in the process (e), while avoiding the influence of the concentration of the impurity on the other region.